Patent Document 1 discloses a driving assistance device that executes a vehicle downhill assist control. The vehicle downhill assist control is one example of the driving assistance control that operates the brake system of a vehicle to adjust the braking force applied to the vehicle such that the vehicle speed does not exceed a target speed. The vehicle downhill assist control is started if the vehicle speed reaches a value greater than or equal to an initiating speed so that an initiating requirement is satisfied in the condition in which a manually-operated activation switch is on.
In the device disclosed in Patent Document 1, if the driver manipulates the accelerator pedal during execution of the vehicle downhill assist control, the device increases the target speed as the manipulation amount of the accelerator pedal is increased. If the target speed becomes greater than the vehicle speed, the device reduces the braking force applied to the vehicle by the operation of the brake system.
Vehicles that do not include a torque converter on the power transmission path from the engine to the wheels include, for example, vehicles that have a manual transmission. In such a vehicle, when braking force is applied to the wheels by executing the vehicle downhill assist control, the load applied to the engine is increased, and the engine speed may be decreased. Such decrease in the engine speed that accompanies executing the vehicle downhill assist control is significant when the gear position of the transmission is in a high-speed gear position.
Such a phenomenon is not limited to the vehicle with the manual transmission, but may similarly occur in a vehicle that does not include the torque converter on the power transmission path as described above.